FAQ:
Antibiotic Resistance and Foodborne Illness
What
is an antibiotic?
What is antibiotic resistance?
Why are bacteria becoming resistant to antibiotics?
What is the Centers for Disease Control and Prevention
(CDC) doing to monitor antibiotic resistance in foodborne bacteria which
may be present in food?
Why is antibiotic resistance a food safety problem?
How do bacteria that are in food become resistant to antibiotics?
Why are antibiotics used in food-producing animals?
Does the use of antibiotics to promote growth pose a public
health risk?
How does antibiotic use in animals differ from use in humans?
How much is used in food-producing animals?
Which antibiotics used in food-producing animals are related
to antibiotics used in humans?
How do resistant bacteria spread from animals to humans?
What is the human health consequence of increasing antibiotic
resistance in foodborne bacteria?
What can be done to slow antibiotic resistance?
What is an antibiotic?
An antibiotic
is a medicine that kills or inhibits the growth of microbes, such as
bacteria and fungi. The term "antibiotic" originally referred
to a natural compound produced by a fungus or another microorganism
that kills bacteria which cause disease in humans or animals. Alexander
Fleming discovered the first antibiotic, penicillin, in 1927. Some antibiotics
may be synthetic compounds (not produced by microorganisms) that can
also kill or inhibit the growth of microbes. Technically, the term "antimicrobial
agent" refers to both natural and synthetic compounds; however,
many people use the word "antibiotic" to refer to both. Although
antibiotics have many beneficial effects, their use has created the
new problem of antibiotic resistance.
What
is antibiotic resistance?
Antibiotic
resistance is the ability of bacteria or other microbes to resist the
effects of an antibiotic. Bacteria can do this through several mechanisms.
Some bacteria develop the ability to neutralize the antibiotic before
it can do harm, others can rapidly pump the antibiotic out, and still
others can change the antibiotic attack site so it cannot affect the
function of the bacteria.
Why
are bacteria becoming resistant to antibiotics?
Antibiotics
kill or inhibit the growth of susceptible bacteria. Sometimes one of
the bacteria survives because it has the ability to neutralize or evade
the effect of the antibiotic; that one bacteria can then multiply and
replace all the bacteria that were killed off. Exposure to antibiotics
therefore provides selective pressure, which makes the surviving bacteria
more likely to be resistant. In addition, bacteria that were at one
time susceptible to an antibiotic can acquire resistance through mutation
of their genetic material or by acquiring pieces of DNA that code for
the resistance properties from other bacteria. The DNA that codes for
resistance can be grouped in a single easily transferable package. This
means that bacteria can become resistant to many antimicrobial agents
because of the transfer of one piece of DNA.
What
is the Centers for Disease Control and Prevention (CDC) doing to monitor
antibiotic resistance in foodborne bacteria?
To monitor
antibiotic resistance, CDC, supported by the Food and Drug Administration
(FDA) and in partnership with 28 state and local health departments
established the National Antmicrobial Resistance Monitoring System for
Enteric Bacteria (NARMS) in 1996. This system detects emerging resistance
and guides studies that evaluate where and how people become infected
with resistant foodborne bacteria. For more information, see the NARMS
website at www.cdc.gov/narms.
CDC and state health departments investigate outbreaks caused by particular
bacteria, and conduct other studies to better understand the circumstances
under which they arise and spread.
Why
is antibiotic resistance a food safety problem?
Antibiotic
resistance is a food safety problem for several reasons. First, antibiotic
resistance is increasing to some antibiotics, such as fluoroquinolones
and third-generation cephalosporins. These antibiotics are commonly
used to treat serious infections caused by bacterial pathogens frequently
found in food, such as Salmonella and Campylobacter. Each
year, several million people in the United States are infected with
Salmonella and Campylobacter, which usually cause diarrhea
that lasts about a week. Antibiotics are not recommended for treatment
of most of these diarrheal illnesses, but are used to prevent complications
in infants, persons with weakened immune systems, and older persons.
Antibiotics may be life-saving for several thousand people each year
who have serious invasive infections, such as bacteremia (infection
in the bloodstream) and meningitis (infection of the lining of the brain
and spinal cord). Salmonella infections are treated with ampicillin,
trimethoprim-sulfamethoxazole, fluoroquinolones or third-generation
cephalosporins, but some Salmonella and Campylobacter
infections have become resistant to these medicines.
A second
reason that antibiotic resistance is a food safety problem is that more
people may become ill. Ordinarily, healthy persons who consume a few
Salmonella may carry them for a few weeks without having any
symptoms, because those few Salmonella are held in check by the
normal bacteria in their intestines. However, even a few antibiotic-resistant
Salmonella in food can cause illness if the person who consumes
the contaminated food then takes an antibiotic for another reason. The
antibiotic can kill normal bacteria in the gut, letting a few Salmonella
that ordinarily would be unlikely to cause illness, take over and cause
illness.
A third
possible reason that antibiotic resistance is a food safety problem
is that the food supply may be a source of antibiotic-resistant genes.
Harmless bacteria present in food-producing animals could be resistant,
and humans could acquire these bacteria when they eat meat products
from these animals. Once ingested, resistant genes from these bacteria
could be transferred to bacteria that cause disease. Quantifying the
extent to which this contributes to a food safety problem is difficult.
How
do bacteria that are in food become resistant to antibiotics?
Many of
the bacteria in food that cause disease are found in the intestinal
tracts of animals or people. Use of antibiotics in food animals and
people can select for resistant strains that end up in the food supply.
Healthy food-producing animals commonly carry bacteria that can cause
illness in humans, including Salmonella and Campylobacter. Humans
are the reservoir for some food-borne bacteria, including Shigella
and Salmonella Typhi (the cause of typhoid fever), and increasing
resistance in these bacteria is predominately the consequence of antibiotic
use in humans.
Why
are antibiotics used in food-producing animals?
Antibiotics
are used in food-producing animals for three major reasons. First, antibiotics
are used to treat sick animals. Second, antibiotics are used in the
absence of disease to prevent diseases during times when animals may
be susceptible to infections. This use affects a larger number of animals,
because it usually involves treating a whole herd or flock, which increases
the likelihood of selecting for organisms that are resistant to the
antibiotic. In these two examples, high doses of antibiotics are used
for short periods of time. Third, antibiotics are commonly given in
the feed at low doses for long periods to promote the growth of cattle,
poultry, and swine. In the 1950s studies showed that animals given low
doses of antibiotics gained more weight for a given amount of feed than
untreated animals. Exactly how this occurs is unknown. This effect appears
to be less effective or absent in animal production settings with good
sanitation.
Does the use of antibiotics to promote growth pose a
public health risk?
The use
of antibiotics to promote growth is widespread in food animal production.
Antibiotics used for growth promotion increase the pressure for bacteria
to become resistant. To address this public health problem, the World
Health Organization (WHO) has recommended that antibiotics not be used
for this purpose. It is determined that this practice is unsafe for
the public's health (World Health Organization).
How
does antibiotic use in animals differ from use in humans?
In humans,
antibiotics are usually used to treat sick individuals but can occasionally
be used to prevent illness. Sick animals are sometimes treated individually,
but often whole flocks or herds of animals are treated at once, including
animals that are not ill. In humans, antibiotics are sometimes given
to healthy persons to prevent specific infections; this type of use
is much more common in animals. In humans, antibiotics are not given
to promote growth, yet this is a major reason for using antibiotics
in animals.
How
much is used in food-producing animals?
In the
United States, data on the quantity of antibiotics given to food animals
is not available to the public or to government agencies. The quantity
used in food animals has been estimated to be 27 million pounds by the
Union of Concerned Scientists (www.ucsusa.org)
and to be 18 million pounds by the Animal Health Institute (www.ahi.org).
Which
antibiotics used in food-producing animals are related to antibiotics
used in humans?
The majority
of antibiotics used in food animals belong to classes of antibiotics
which are also used to treat human illness; these include tetracyclines,
sulfonamides, penicillins, macrolides, fluoroquinolones, cephalosporins,
aminoglycosides, chloramphenicols, and streptogramins. Because these
classes of antibiotics are similar, then bacteria resistant to antibiotics
used in animals will also be resistant to antibiotics used in humans.
How do resistant bacteria spread from animals to
humans?
Resistant
bacteria may be transferred to humans through the food supply or direct
contact with animals. For example, Campylobacter lives in the
intestines of chickens. People get Campylobacter diarrhea primarily
from eating undercooked chicken. In 1989, none of the Campylobacter
strains from ill persons that CDC tested were resistant to fluoroquinolone
antibiotics. In 1995, the FDA approved the use of fluoroquinolones in
poultry. Soon afterwards, doctors found that Campylobacter from
ill persons were resistant to fluoroquinolone antibiotics.
What
is the human health consequence of increasing antibiotic resistance
in foodborne bacteria?
When an
ill person is treated with an antibiotic to which the bacteria is resistant,
the antibiotic will not help and may even make the illness worse. Also,
increasing antibiotic resistance in the bacteria harbored by animals
makes it more likely for humans who do get infected to have a resistant
strain. The illness may last longer, be more serious, or more expensive
to treat.
What can be done to slow antibiotic resistance?
Decreasing
unnecessary or imprudent antibiotic use will decrease the pressure on
organisms which are exposed to them to become resistant. Ongoing efforts
in human and veterinary medicine are needed to decrease the misuse and
overuse of antibiotics, so that the efficacy of antibiotics is preserved
for as long as possible. For example, medical and veterinary professional
organizations have issued recommendations to promote appropriate therapeutic
use of antibiotics by physicians and veterinarians. A Task Force of
11 government agencies issued a Public Health Action Plan to Combat
Antimicrobial Resistance in 2001, and a progress report on implementation
of the plan in 2002 which are available at www.cdc.gov/drugresistance.
In addtion
a brochure containing some commonly asked questions concerning NARMS
can be found at FDA's
Center for Veterinary Medicine.